Method and means for communication



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METHOD AND MEANS OF COMMUNICATION Filed Feb. 6, 1945 I SSheets-Sheet 5 INVENTORS EMILE LflB/N DO/VHLD D. GR/EG Patented Oct. 29, 1946 METHOD AND MEANS OF COMIVIUNICATION Emile Labin, New York, and Donald D. Grieg,

Forest Hills, N. Y., assignors to Federal Telephone and Radio Corporation, Newark, N. .L, a

corporation of Delaware Application February 6, 1943, Serial No. 474,960

9 Claims.

This invention relates to improvements in methods and means of communication, and more particularly to arrangement for improving the selection and preventing interference between modulated pulse carriers having a pulsing frequency difference between them.

In the case of so-called multiplex communication systems, attempts have been made, for example, to use a single channel for transmitting intelligence from several sources by separate intelligence-modulated pulse trains which have a very slight frequency difference between them. Also, it may be desirable, at times, to transmit intelligence by means of pulsed high frequency carriers which differ from one another in pulsing rate by a slight amount. It is now possible, in either of these cases, to substantially eliminate the undesired signal per se but interference still results from the overlapping of certain pulses or high frequency carriers, causing sets of peaks. While these peaks themselves, which occur at more or less regular intervals, could be eliminated by suitable filters, a large amount of interference will still exist due to the signal modulation producing complex beats.

The primary object of the present invention is to provide a method and means applicable in the above-mentioned cases for eliminating the interference caused by such complex beats and thus permitting the selection of a clear desired signal.

Another object of the present invention is directed to the provision of a system for the transmission of intelligence by means of which a plurality of signals may be transmitted over a single channel which may be either wire or wireless, and to a receiver or a plurality of receivers each of which may pick out a desired signal with a minimum of interference.

The principles of the present invention are also applicable to arrangements for receiving signals through jamming or similar interference with a minimum loss and a maximum clarity.

Other objects and advantages of the present invention will appear from the following description of a specific embodiment and modifications thereof, in the accompanying drawings in which:

Fig. 1 is a block circuit diagram illustrating a preferred system according to the present in vention;

Fig. 2 is a series of curves illustrating the operation of the system illustrated in Fig. 1;

Fig. 3 is a second series of curves illustrating the adaptation of the system of Fig. 1 to another type of intelligence transmission.

As shown in Fig. 1, there may be provided a plurality of signal sources of which two, In and i2 are illustrated. In accordance with the first form of the invention, the signal sources may consist of separate sub-carriers in the form of trains of modulated pulses, the frequency of one train differing slightly from the frequency of the other train. This frequency difference may be cycles or less, or up to 1,000 cycles where, for example, the frequency of each pulse train is in the neighborhood of 12,000 cycles. The two (or more) pulse trains may be transmitted over a common channel It which may either be wired, or may, for example, be a common carrier wave. The receiver or receivers of which two, I6 and I0 are illustrated are connected or tuned to this common channel It so that they will receive both sets of pulses. In Fig. 2 (I) we have indicated a series of pulses a which may be derived from source A and at (II) a series of pulses 7) derived from source B. At the receiver I5, for example, these two trains of pulses of different frequency will produce a pulse signal somewhat as indicated in Fig. 2 (III). It will be noted that due to the slight frequency diiference, at certain intervals some of the pulses will overlap, producing peaks which, generally speaking, will be the sum of the individual overlapped pulses. At other points the two sets of pulses will be either adjacent. one another or more or less spaced apart. In accordance with the present invention, these incoming pulses are first impressed upon a first mixer l8 which is controlled by a blocker 20 whose wave may have somewhat the shape indicated in Fig. 2

(IV) so as to prevent the passing through of all pulses except those corresponding to the frequency of the pulse train from source A, allowance being made, of course, for pulse displacements or other changes due to modulating signals. When this blocking device 20 is applied to the mixer Hi, the resulting output in line 22 will be somewhat as indicated in Fig. 2 (V). It will be seen that all individual undesired pulses such as the pulses b have been eliminated but this does not eliminate the peaks or partial peaks caused by overlapping of the pulses b with the pulses it. Since these periods of overlapping will occur at predetermined intervals in accordance with the frequency difference between pulse trains, it would be possible to eliminate the peaks by suitable filters. However, in addition to the peaks themselves, the overlap between the two sets of pulses causes complex beats due to the modulation of the individual pulses, and these beats cause undesirable interference in the received intelligence. Accordingly, by the present invention, the output from lead 22 of the receiver is directed to a second blocking device 24 which may consist of a clipper 28 which takes off the top portions of the peaks reversing them by means of a shaper 30 and then applies them in a second mixer 26 to entirely eliminate those portions of the a pulses which are overlapped. The result of this operation is indicated in Fig. 2 (VI), wherein it can be noted that the first of the series of pulses illustrated, has now been entirely omitted, while the second pulse has been out in half by the elimination of its overlapping pulse. The remaining a pulses are not afiected. The fact that one of the retained pulses is smaller than the other pulses is of little consequence. It will produce no interference: if the-pulses are differentiated and the intelligence is controlled by the leadin edge of the retained pulses. In

Fig. 1, we have shown the output lead 30 from the second mixer 26, applied to a differentiating circuit 32, the output 34 from which. will have a wave shape of differentiated pulses a as indicated in Fig. 2 (VII).

The absence of a desired pulse indicated by the blank in the first line of Fig. 2 (VII) may produce a. low audible beat in the finally received signal. This can readily be eliminated by passing the received signals through a low-frequency rejection filter. In Fig. l, we have illustrated a demodulator 36 connected to the output lead 34. The output from this demodulator being fed through the rejection filter 38. The signal output from the filter 38 will, therefore, be substantially free of any interference caused by the simultaneous reception of a plurality of pulse trains.

The other receivers such as [6' will be similar to the receiver l6, although they may be synchronized in time, for example, to the signals from source B. In such a case the blocker 20 would act upon the first mixer 18' to eliminate all but the b pulses. The second block 24, the second mixer 26, the difierentiating circuit 32', the demodulator 36' and the low-pas filter 38 acting aspreviously described in connection with the receiver [6. c

It is to be understood that while we have illustrated two sources of signals and two receivers, the invention is applicable to arrangements including any number of signal sources in connection with one or more receivers. It will also be clear that provision may be made at each receiver for varying the first blocking device (20) to permit controllably picking out of the desired signal from any other undesired signals.

The principles of the present invention are also applicable to prevent interference between a desired high frequency carrier and an undesired high frequency carrier having the same high frequency carrier frequency but differing in pulsing rate or sub-carrier frequency. This second application is graphically illustrated in Fig. 3 in which line (VIII), shows for example, the upper half of a desired pulsed carrier wave a: while line (IX) similarly shows an undesired pulsed carrier 3 At certain intervals the two sub-carriers will progressivelyinterfere as indicated in line (X). If the receiver is periodically unblocked to eliminate all but the desired subcarrier 1:, the result will be as is shown in lines been distorted by interference and, in an actual case, there will be series of such distorted subcarriers as illustrated in the first three examples, in line (XII). For the purpose of eliminating all such distorted sub-carriers in this case, the second blocker will be initiated by a clipper, as before, but the blocking pulse will have a time lag which covers all of the distorted sub-carriers. This may be accomplished, as will be clear to those skilled in this art, by means of a suitable multivibrator circuit producing a blocking pulse as shown in line (IHII), The resultant signal will then be represented as in line QGIV) by pulsed sub-carriers a: between which, at certain intervals, a larger void will exist. However, this periodic absence of signal causes less interference than is caused by mere elimination of the undesired sub-carrier and the low frequency beat note which may result from the periodic absence of signal can be eliminated by passing the signals through a suitable low-pass rejection filter.

While the methods and means of the present invention are particularly applicable to systems of multiplex communication, it will also be seen that they provide an arrangement for eliminating interference due to jamming, if the jamming signal is not exactly synchronized with the intelligence to be received. Thus, returning to Fig. 1, if the source B were a modulated jammin pulse rather than a second signal source, the adverse effects of such jamming pulse will be substantially eliminated in the same manner as described in eliminating the undesired signal.

Throughout the present description and claims, the'term sub-carrier has been used in a broad sense to mean any periodic wave or pulse which is modulated by the signals to be transmitted. For example, where a pulse is itself modulated in accordance with intelligence, the pulse forms the sub-carrier, in other cases the sub-carrier may he a high frequency wave which is pulsemodulated in accordance with signals to be transmitted. I In contrast, in the former case a second common radio frequency carrier or channel may be used -to carry the modulated pulses, but this is not the interfering "sub-carrier to which reference is made, as this carrier merely acts as the common channel.

While we have described certain features of the present invention which are deemed essential to a complete understanding thereof, and while we have illustrated and described the invention in connection with certain specific embodiments, it is to be understood that the description is made only by way of example and not as a limitation on the scope of our invention as set forth in the objects and the accompanying claims.

We claim: a

l. The method of selecting a desired modulated pulse train from among a plurality of modulated pulse trains having a slight frequency difference relatively to the desired pulse train, which includes the steps of blocking out the undesired pulses whenever they do not overlap the desired pulses, and blocking out at least the pulse portions of those desired pulses which are overlapped by the undesired pulses.

2. The method according to claim 1, which includes the further step of eliminating the beat note caused by the absence of desired pulses.

3. The method for the transmission and reception of intelligence which includes the steps of simultaneously transmitting a plurality of intelligence modulated pulse sub-carriers having slightly different pulse repetition rates substantially simultaneously receiving these sub-carriers at a plurality of receiving stations, and at each receiving station selecting the desired intelligence by blocking out the undesired sub-carriers and blocking out those portions of the desired subcarrier which have been distorted by interference with the undesired sub-carriers.

4. In a selecting system for receivers adapted to receive a plurality of pulse sub-carriers havstage for periods corresponding to the frequency of the desired pulse train, means for clipping the pulses of the desired train which have been peaked by overlapping with undesired pulse trains, and further blocking means controlled by the corresponding peaks for blocking out at least the pulse portions of those desired pulses which were overlapped by the undesired pulses.

6. The combination according to claim 5, in combination with pulse difierentiating means 6 producing a signal in accordance with at least one of the edges of the desired pulses.

7. The combination according to claim 5, in combination with pulse difierentiating means producing a signal in accordance with at least one of the edges of the desired pulses, and a rejection filter for eliminating the beat note resulting from the periodic absence of signal of the desired pulse train caused by the last-mentioned blocking means.

8. A system of communication including in combination a plurality of transmitters, each transmitting intelligence over pulse sub-carriers difierentiating only in slightly different pulse rates, means providing a common transmitting channel for simultaneous transmission of the pulse sub-carriers of said transmitters, and one or more normally blocked receivers coupled to said common channel means, each receiver including unblocking means permitting reception of a desired sub-carrier, and blocking means for blocking out at least those portions of the desired sub-carriers distorted by the undesired subcarriers.

9. The combination according to claim 5, in combination with a rejection filter for eliminating the beat note resulting from the periodic absence of signal of the desired pulse train caused by the said last means.

EMILE LABIN. DONALD D. GRIEG. 

